Low stress electrodeposited rhodium



atent 2,895,889: Patented July 21, 1959 LOW STRESS ELECTKODEPOSITED RHODIUIVI Karl Schurnpelt, Union, N .J assignor to Sal-Rex Precious lIVIetals, Inc., Belleville, N.J., a corporation of New ersey No Drawing. Application January 10, 1957 Serial No. 633,381

Claims. (Cl. 204-47) This invention relates to a process for obtaining electrodeposited rhodium layers which have a low stress and to the bath for obtaining such low stress deposits.

Internal stress in metallic deposits causes the deposit to crack, peel or distort. Shrinkage in a highly stressed deposit may cause premature fatigue failure of the basis metal. Where it is known that a metal deposit is likely to be plated under stress a severe etching of the basis metal before plating is usually essential in order to obtain a satisfactory product. It is desirable, therefore, to be able to plate a metal such as rhodium under low stress but little is known as to the exact cause of internal stress in electrodeposited metals.

Among the objects of this invention is to provide an electroplating composition capable of plating rhodium metal under low stress.

Among other objects of the invention is to provide a body having a rhodium deposit under low internal stress.

Among other objects of the invention is to provide a process of plating rhodium under low stress.

A fairly comprehensive study of rhodium plating was made by Wiesner and reported in 39th Annual Proceedings and Technical Sessions, American Electroplaters Society (published in 1952), pp. 79-99. According to this article the addition of a small amount of chloride ion to a rhodium electroplating bath (which may be the rhodium sulfate or the complex rhodium phosphate bath) reduces the stress of the deposit. The chloride ion also causes a reduction in brightness. If the chloride ion exceeds a certain minimum amount of about 0.3 g./l. the deposit obtained is rough. Wiesner also found that operating the baths at higher temperatures (3854 C., for example) reduced the stress in the deposit. Common organic addition agents such as saccharine employed for reducing stress of deposits from nickel baths were found to reduce the cathode efliciency a prohibitive amount.

The objects of the present invention are obtained by providing a rhodium sulfate-sulfuric acid bath and add ing to this bath to 100 g./l. of magnesium sulfate (MgSO The cathode efliciency of a rhodium sulfate sulfuric acid bath may be as high as about 88-90%. After the addition of magnesium sulfate the efficiency remains high. Instead of having an internal tensile stress tending to pull the rhodium film away from the basis metal the deposits show a tendency to curl toward the base indicating that the rhodium deposit is under compressive stress.

The free sulfuric acid content of the bath is from 20 to 100 cc./l. Very satisfactory results are obtained with 50 cc. of free sulfuric acid per liter.

The rhodium sulfate content of the bath is such as to provide about 2 g. to 5g. of rhodium per liter. Higher amounts of rhodium may be employed but with such higher amounts drag-out and other losses are increased without increasing the efliciency.

The bath may be operated from room temperature (approximately 20 C.) to about 50 C. and at crnrent densities of from 4 to 20 amperes per square foot of cathode. The higher current densities produce slightly more stress. Plating at about 10 amperes per square foot and 50 C. with mild agitation yields a good deposit at about cathode efliciency.

A convenient qualitative test to determine stress in the deposit is to dissolve away the basis metal from a rhodium deposited body with nitric acid for example. If the basis metal is dissolved from an article having a rhodium film deposited from the common complex rhodium phosphate bath the rhodium disintegrates into small flakes. This indicates that the deposit has high internal stress.

The following examples further illustrate the principals and operation of the invention.

Example 1 A sheet of metal, for example brass, is made the cathode in an electroplating bath comprising 5 g./l. of rhodium (as rhodium sulfate) 50 cc./l. of sulfuric acid, and 50 g./l. of magnesium sulfate. The bath is maintained at 50 C. and operated at a current density of approximately 10 amperes per square foot until a thickness of about 0.0003" is obtained. The deposit has a slight grey appearance but is smooth. The efiiciency of the bath is 85%. Spectrographic examination of the plated article did not indicate the presence of any magnesium. The basis metal could be dissolved away from this prodnet to leave a thin continuous film of rhodium.

The features and principles underlying the invention described above in connection with specific exemplifications will suggest to those skilled in the art many other modifications thereof. It is accordingly desired that the appended claims shall not be limited to any specific feature or details thereof.

I claim:

1. A process of electroplating rhodium metal under low internal stress comprising electrodepositing rhodium upon a basis metal from an aqueous bath consisting essentially of 20-100 cc. per liter of free sulfuric acid, 2 to about 5 grams per liter of rhodium added as rhodium sulfate and 10-100 grams per liter of magnesium sulfate, the remainder being water, electrodepositing rhodium from said bath at a current density of from 4 to 20 amperes per square foot and at a temperature of about 20 C. to about 50 C.

2. The process as set forth in claim 1 wherein said bath contains 50 cc. per liter of sulfuric acid and 50 grams per liter of magnesium sulfate.

3. An electroplating bath for depositing rhodium therefrom comprising an aqueous bath consisting essentially of 20-100 cc. per liter of free sulfuric acid, 2 to 5 grams per liter of rhodium added as rhodium sulfate, 20-100 grams per liter of magnesium sulfate and the remainder being water.

4. The bath as claimed in claim 3 containing 50 cc. per liter of sulfuric acid and 50 grams per liter of magnesium sulfate.

5. An electroplated article comprising a basis metal and a substantially stress-free electrodeposited layer of rhodium thereon, said article having been prepared by the process of claim 1.

References Cited in the file of this patent UNITED STATES PATENTS Shields Feb. 27, 1934 OTHER REFERENCES 

1. A PROCESS OF ELECRROPLATING RHODIUM METAL UNDER LOW INTERNAL STRESS COMPRISING ELECTRODEPOSITING RHODIUM UPON A BASIS METAL FROM AN AQUEOUS BATH CONSISTING ES-SENTIALLY OF 20-100 CC. PER LITER OF FREE SULFURIC ACID, 2 TO ABOUT 5 GRAMS PERLITER OF RHODIUM ADDED AS RHODIUM SULFATE AND 10-100 GRAMS PER LITER OF MAGNESIUM SULFATE, THE REMAINDER BEING WATER, ELECTRODEPOSITING RHODIUM FROM SAID BATH AT A CURRENT DENSITY OF FROM 4 TO 20 AMPERES PER SQUARE FOOT AND AT A TEMPERATURE OF ABOUT 20* C. TO ABOUT 50* C. 